The present invention relates to the field of magnetic recording, and more precisely to the magneto-electric transducers which are used therein.
In this field, use is made of a magnetic support medium in the form of a tape of floppy disks, covered with a layer of ferro-magnetic material which forms the active part thereof.
This recording medium is moved in relation to a transducer element, comprising a magnetic circuit in ring form interrupted by a very narrow air-gap, of a length typically between 1 and 10.mu., an electric circuit, formed by windings in the form of coils, surrounding the magnetic circuit.
Two functions may be assumed by such a transducer. If an alternating electric voltage is applied to the windings, a magnetic flux is created in the magnetic circuit, determining an opening-out in the art of the lines of force of a magnetic field at the air-gap; the mobile support medium, which moves in front of this latter, is thus permanently magnetized following a law of variation identical to that of the voltage applied to the transducer. This latter thus assumes the function of recording transducer or "head", and that whatever the speed of the magnetic support medium.
But in many applications, it may assume a second function: if the magnetized tape is passed a second time in front of the air-gap, it creates a variable magnetic flux in the magnetic circuit, by a phenomenon symmetrical to the preceding one, according to a law of variations identical to that of the voltage applied to the transducer in the recording phase; the transducer thus assumes the function of reading "head", after the flux variations have been converted into voltage variations by a suitable means.
Several physical phenomena have already been used to achieve this conversion.
The simplest consists in using the same windings as for the recording, and in collecting the voltages developed at their terminals, according to the laws of electromagnetism. A serious disadvantage is however attached to such use, related to the fact that the voltages obtained e are not proportional to the magnetic flux 0 itself, but to the first derivative thereof with respect to time: EQU e=-k(d0/dt)
where k is a constant.
The voltage collected e depends then on the speed of travel of the magnetic support medium; it diminishes therewith and is cancelled out when the medium stops.
This is why use is made of another physical phenomenon, that known under the name of Hall effect and which is the appearance of a variable resistance of certain materials comprising particularly arsenic, indium and/or phosphorous, such as In As P, depending on the magnetic field which is applied thereto; the law of variation of this magneto-resistance is in fact linear depending on this magnetic field. The reading signal obtained has then an output level independent of the travelling speed of the support tape or disk of the recording, including complete stopping of this support medium.
A basic limitation is however attached to use of the Hall effect in the field of "heads": this is the fact that, like all the phenomena using a resistance, this effect is not reversible and that, accordingly, the Hall effect transducer does not allow recording, but only reading. The result of these limitations is that magnetic recorders, since they must operate in the field of control instruments for example, must be fitted with two distinct heads, one serving for recording, of the electro-magnetic type with coiled windings, the other for reading, with a magneto-resistance in the form of a block disposed in the flux of the magnetic circuit, usually in an air-gap of this circuit, provided for this purpose.
This requirement of two distinct transducers constitutes a serious disadvantage in the uses; it is one of the aims of the present invention to propose a single magnetic head assuming the two functions by association, in a single magnetic circuit, of a winding for recording and of a magneto-resistance for reading.
It is to be noted that the originality of the invention resides not only in the union of the two respective means described but also in their combination with the magnetic circuit, thus allowing, as will be shown hereafter, common aims sought after to be reached, by adaptation and particular localization of these means in this circuit.
In fact, as is described and explained further on, the use of a magneto-resistance is not made therein in the magnetic flux which flows in the magnetic circuit, as in the known art, an arrangement which requires an interruption in this circuit, with introduction of a harmful reluctance; on the contrary, in accordance with the invention, the magneto-resistance is therein placed in the magnetic leakage flux of the air-gap, which preserves the optimum conditions of design and operation of the magnetic head. Other advantages attached to this particular localization and derivated therefrom are described further on.
Another limitation exists, during the recording phase, in magnetic heads with windings of the known type, i.e. the non controllable behaviour of the magnetic circuit, depending, on the one hand, on the value of the induction which is created therein by the windings and, on the other hand, on the frequency of the electric signal which is applied thereto.
The value of the induction governs therein the notion of linearity and saturation to high levels, and that of the frequency governs the notion of losses, either by hysteresis and eddy currents in the magnetic material of the circuit, or by variation in the geometric form of the lines of force of the magnetic field in the vicinity of the effective air-gap.
Now, the possible use of an additional winding around the magnetic circuit would not allow the magnetic state of the circuit to be known, either absolute value, or in relative value, for the reasons already given above.
Furthermore, the monitoring winding would behave like the secondary of a transformer whose primary was formed by the recording winding, and the information collected about the magnetic state of the circuit would be completely false.
On the contrary, the magneto-resistance present in the transducer of the invention allows this magnetic state to be known directly as an absolute value during the recording phase.
Another aim of the present invention is to provide a magneto-electric transducer for magnetic recording comprising an element, with magneto-resistance, for permanently monitoring the magnetic state of the circuit of the recording head, in particular for monitoring the correct progress of the recording.
Furthermore, in a certain number of practical uses, a proportionality, or "linearity" as good as possible, is sought for between the signal applied, on recording, to the windings of the magnetic head, and the magnetic field reigning in the effective air-gap, in which, as was pointed out above, only a part of the lines of force opens out in the air to magnetize the magnetic support medium which travels, in contact therewith, in front of the active face of the head.
Now, this effective magnetic field is impossible to measure with a magnetometric means, the presence of the magnetic support medium and of the mechanical parts ensuring its proper travel being totally opposed thereto.
On the other hand, the inner side of the ring forming the magnetic circuit lends itself to the placing of a magnetometric measuring element and, in accordance with the invention, use is made of a Hall effect magneto-resistive element for thus accurately knowing the value of the effective magnetic field delivered by the air-gap, that of the internal side being proportional thereto.
The signal supplied by the measuring element may then be advantageously used for linearizing the effective field, for example by using a feedback loop circuit controlled by the magneto-resistive element.
A further aim of the invention is to provide a high linearity recording system comprising an amplifier and a magnetic head in accordance with the invention, the amplifier receiving at its input terminals the signal to be recorded, its output terminals being connected to the windings of the magnetic head, an adjustable value feedback loop being formed between the terminals of the magnetio-resistance of the magnetic head and the input of the amplifier, thus ensuring linearization of the magnetic field of the air-gap of the magnetic head with respect to the signal applied to the windings.